A number of countries has legislation containing maximum limits for the emission of nitrogen oxides (NOX) in exhaust gases of vehicles driven by combustion engines. A method to reduce the amount of NOX in the exhaust is the use of selective catalytic reduction (SCR). In SCR systems, NOX in the exhaust gas are converted via a reductive chemical reaction into nitrogen and water. The conversion involves a catalytic reaction using a reductor. Ammonia is frequently used as reductor. The ammonia is obtained by disintegration of a precursor solution containing ammonia or urea. The precursor liquid is injected in the exhaust gas flow. To this end a car has one or more tanks containing the precursor liquid. An example of such precursor liquid is AdBlue (trademark of VDA, Verband der Automobilindustrie), commercial name of a water based solution containing 32.5% urea. Other examples are urea/ammonia formate solutions (e.g. sold under the commercial name Denoxium). Such precursor solutions are very corrosive.
Most water based precursor solutions can freeze at temperatures that occur in nature (e.g. a water based solution of 32.5% urea freezes at minus 11° C.). Therefore a system, e.g. a heating element, is required to thaw the precursor tank when the precursor liquid in the tank is frozen. The heating element can also be used to heat the precursor liquid to a suitable operational temperature.
Heating systems to be immersed in SCR tanks have to answer a number of requirements. A first requirement is that the heating systems must be able to generate a sufficient amount of heat to be able to thaw and/or heat in a sufficiently short time period the required amount of liquid. The required amount of heat depends on the volume of the tank, and in particular on the maximum amount of precursor liquid the tank can contain. The dimensions of the heating element have to be limited to leave sufficient space in the tank for the tank to contain a sufficient volume of precursor liquid without the tank and the accessories in and around the tank becoming to voluminous. It is requested that heating is performed first around the pump that is pumping the precursor liquid out of the tank, but other sections of the tank also have to be thawed and heated. Preferably, the heating system can heat different sections of the tank (e.g. remote sections in the tank or sections that are physically separated from other sections of the tank). Thawing should not only be performed around the pump, but also in other sections, such that precursor liquid is available at and towards the pump. The tank can have a complex design, with different accessories provided in the tank (pumps, sensors . . . ). Heating systems immersed in the tank are very efficient in terms of transfer of energy (heat) to the (frozen) precursor liquid. However, the immersed heating system should also be resistant to the conditions in the tank: the corrosive precursor (or volatile components, such as ammonia) and temperatures that in use can go up to over 100° C. Required is a long life time of the heating element, without change over time of its characteristics (including generation of heat). Electrical (resistive) heating systems have to be energy efficient, and have to be able to operate without demanding (too) large amounts of energy from the battery of the vehicle, which is especially of importance when starting the vehicle in cold weather conditions. Manufactures of SCR systems prefer heating systems that are easy to install.
A number of different systems have been developed to thaw and heat the tank. Some systems comprise separate resistive heating elements or a bypass of hot liquid or of hot gas around or through the tank. In general the heating systems are complex and expensive. Use of electrical resistive heating elements in the tank is known. EP 1582732 discloses such a system. Another example is provided in WO2008/023021, where a urea tank and a supply hose are described wherein a heating wire is partly provided in the supply hose. The heating wire is in the shape of a spiral or loop that is at least partly present in the tank or in the supply hose.
EP 2339138A1 describes a flange for holding a quantity of fluid within a tank. The flange is equipped with a heating element comprising at least one resistive wire for conducting an electrical current and dissipating heat as an effect of the electrical current. The at least one resistive wire is guided at a plurality of separate locations along its length by guiding means to form a heating surface, wherein a first part of the heating surface is present inside the flange and a second part of the heating surface is present outside the flange.
It is a disadvantage of systems using a heating wire, that the heating wire needs to have at least a diameter of several millimeter. Such thickness makes it difficult (or even impossible) to bend the heating wire to position it in the tank such that the full tank can be heated. Furthermore, such heating wire cannot be positioned at or near accessories of the tank or in remote sections of the tank due to the high resistance of the heating wire to bending.
It is a specific problem of existing heating system that it is difficult or even impossible to position the heating system in (remote) corners of the tank. Such existing systems are complex, and their assembly and installation is time consuming, difficult and expensive.
If the tank gets too hot, it can get overheated. It is desired that the heating systems contain safety mechanisms in order to prevent overheating.
A specific problem relates to the life time, as the conditions of the heating tank are very corrosive.